Multimegameter-range acoustic data obtained by bottom-mountedhydrophone arrays for measurement of ocean temperature

Acoustic signals transmitted from the ATOC source on Pioneer Seamount off the coast of California have been received at various sites around the Pacific Basin since January 1996. We describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source. Stable identifiable ray arrivals are observed in several cases, but some receiving arrays are not well suited to detecting the direct ray arrivals. At 5-Mm range, travel-time variations at tidal frequencies (about 50 ms peak to peak) agree well with predicted values, providing verification of the acoustic measurements as well as the tidal model. On the longest and northernmost acoustic paths, the time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability. An annual cycle is not evident in travel times from shorter acoustic paths in the eastern Pacific, though only one realization of the annual cycle is available. The low-pass-filtered travel times are estimated to an accuracy of about 10 ms. This travel-time uncertainty corresponds to errors in range- and depth-averaged temperature of only a few millidegrees, while the annual peak-to-peak variation in temperature averaged horizontally over the acoustic path and vertically over the upper 1 km of ocean is up to 0.5°C     

Reciprocal acoustic transmissions: Instrumentation for Mesoscale monitoring of ocean currents

By simultaneously transmitting acoustic pulses in opposite directions between two points in midocean, one can separate the effects of ocean currents on acoustic propagation from the effects of sound-speed structure. Reciprocal acoustic transmissions can therefore be used to measure ocean currents. Acoustic transceivers have been designed and built to measure the mean currents between two points separated by 300 km. The equipment functioned satisfactorily during a sbort test conducted during 1983. Preliminary analysis of that experiment has yielded differential travel times that appear reasonable, but more work is required to relate the differential travel times to meaningful ocean-current estimates.